Step 1: Start from the center and work your way out.
Load your brush with a saturated pink. I used a blend of Cotman Permanent Rose and Dr. Ph. Martin's Hydrus Quinacridone Magenta. (For a full list of all my supplies, please read this Getting Started tutorial.)
In a circular motion working away from center, pull the brush around in a swirly motion, twisting the brush slightly, working slowly, and varying the pressure on the bristles to create a varied look. Go slow and practice this motion! Try to do it all without stopping or picking up your brush.
Since rose petals are more compact toward the center of the flower, the petals there will be smaller. So use the tip of your brush to create thinner lines.
You may want to drop a bit more color into the very center while the paint is still wet.
Step 2. Rinse the brush and work outward.
As you work your way outward, remember that the petals get lighter, so rinse the brush out slightly before moving on.
Start at the bottom edge of the swirly center you just created and, with a wet brush, use the side of the bristles to make a flatter, larger petal. Repeat that motion as you add petals around the center of the flower.